The present invention relates to a trapped bushing seal for a rotary member such as a centrifugal compressor. More particularly, the present invention relates to a bushing used in the trapped bushing seal.
Trapped bushing seals are typically used on horizontally-split, barrel-type centrifugal, single stage and axial compressors for installations requiring positive process gas sealing. These seals can be used in hydrocracking, catalytic reforming, refrigeration, or methanol, ethylene, ammonia synthesis as well as other gas compression processes. Compressors utilized in such installations may be operated at speeds from 1,500 to 16,000 rpm or greater.
The problem of providing a seal between a rotating shaft and a stationary housing, such as that found in a compressor, is extremely complex. A particularly difficult situation arises when the process fluid is a caustic or explosive gas. Since in this type of situation it is necessary to insure complete sealing of the gas, liquid seals such as trapped bushing seals utilizing stepped bushings are most frequently used. For ease of explanation, this description will refer to the process fluid as a gas and the sealing fluid as a liquid. However, it should be understood that the invention disclosed may be used in other applications so long as the sealing fluid has a greater density than the process fluid.
If the gas is to be sealed from the atmosphere, the pressure of the sealing liquid entering the seal must be at a greater pressure than the gas pressure. If the sealing liquid pressure is too high, an excessive amount of the liquid will flow toward the process gas where it may become contaminated, and an expensive separation procedure may be necessary to remove the gas from the sealing liquid or the liquid may have to be discarded. Therefore, attempts have been made to maintain the liquid pressure only slightly higher than the gas pressure so an excessive amount of the sealing liquid does not become contaminated by the gas. However, it has been found that if two small a pressure differential is maintained, gas becomes entrained in the sealing liquid during operation and will find its way to atmosphere. This entrainment is due to differential pressure patterns that are set up in liquid passages within the seal. These differential pressure patterns are caused by conventional Journal bearing action between the impeller and the stepped bushing. As the impeller rotates, it squeezes the sealing liquid against the bushing causing high and low pressure areas which vary in intensity depending on the eccentricity between the relative roundness of the impeller and the stepped bushing. These pressure areas may rotate or assume some relatively fixed position In the sealing passages. If the liquid pressure in these low pressure areas is below the gas pressure, the gas will enter the low pressure areas and find its way to atmosphere. In order to insure that the gas does not enter the low pressure areas, the sealing liquid pressure has to be sufficiently high so that the pressure of the liquid in the low pressure areas is greater than the pressure of the gas, but not so high that the sealing liquid flows toward the process gas.
It has been discovered that when the rotating impeller is running at relatively high rpm the bushing of the trapped bushing seal may develop "cross coupling" to destabilize the rotating element. It is believed this cross coupling is a result of the inability of the bushing to track the movement of the impeller and shaft thus resulting in a subsynchronous vibration. In the past, such vibrational problems were solved with larger clearances in the seal and multiple circumferential grooves in the bushing. However, both of these modifications to the bushing have drawbacks. For example, larger clearances in the seal result in undesirable higher flow of the sealing fluid, and the use of circumferential grooves reduces the load carrying ability of the bushing making it more susceptible to "rubs".